1. Field of the Invention
The present invention generally relates to a metal halide lamp and, more particularly, to contents which are contained in a hermetically sealed tube of a metal halide lamp.
2. Related Background Art
A metal halide lamp is a lamp in which a metal halide is added in a sealed tube, in which mercury vapor is contained at a high pressure, to improve the luminous efficacy and color rending properties, and is widely used for general illumination. A conventional metal halide lamp is fabricated by charging, in a light-transmitting quartz tube, an inert gas, e.g., argon (Ar), at least one kind of halide (LnX.sub.2 or LnX.sub.3 : where Ln is a rare earth metal, e.g., scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu), and X is bromine (Br) or iodine (I)), mercury (Hg)), and an iodide (NAI: where NA is an alkali metal, e.g., sodium (Na), lithium (Li), cesium (Cs), potassium (K), or rubidium (Rb)) and sealing the tube. It should be noted that bromine or iodine, the rare earth metal and the alkali metal will be generally represented by symbols "X", "Ln" and "NA", respectively, hereinafter and the attached drawings.
In the above conventional metal halide lamp, tungsten (W) as the base material of electrodes is liberated by sputtering during use, and free tungsten reacts with silicon dioxide (SiO.sub.2) as a constituent component of the sealed tube to deposit on the inner wall surface of the sealed tube, thereby blackening the tube wall within a short period of time. Blackening of the tube wall decreases the luminous efficacy and lumen maintenance factor. When the lumen maintenance factor decreases to about 70%, the metal halide lamp becomes inappropriate for practical use.
To solve this problem, a means of adding bromine in the sealed tube so that bromine is in excess of the rare earth metal is proposed (Japanese Patent Laid-Open No. 55-32338). According to this means, an excess of bromine reacts with free tungsten during electric discharge to form a compound (WBr.sub.2 and WBr.sub.3), thereby suppressing reaction of silicon dioxide of the sealed tube with tungsten. However, since bromine also reacts with mercury, free tungsten remains, which will likely be deposited on the inner wall surface of the sealed tube. Thus, the sealed tube is blackened within a comparatively short period of time even if an excess of bromine is added.